Resistance welding is a thermo-electric process in which heat is generated at the interface of two parts to be joined by passing an electrical current through the parts for a precisely controlled time and under a controlled pressure. The power for the resistance welding process is provided by a power supply unit. The resistance welding power supply unit can transform, modulate and control the electrical energy received from a power supply network and applies it to the resistance welding equipment according to a weld schedule. The resistance welding equipment can comprise a weld head comprising welding electrodes. The welding electrodes are installed in the weld head to touch and maintain contact with the workpieces according to the weld schedule. The weld head including the welding electrodes functions to force the workpieces together and to hold them during the welding process. The amplitude and duration of forces and heating parameters are defined by the weld schedule. The weld schedule can comprise different parameters including an electrode force, a squeeze time, and a hold time.
Welding transformers are used in the power supply unit to change an alternating current from the power supply network into a low voltage, high amperage current in the secondary winding. Inverter-based power sources have replaced traditional thyristor-controlled power sources. A switch mode converter-based power source or power supply unit comprises an inverter bridge circuit. The inverter bridge circuit can be controlled using a pulse width PWM scheme to regulate the output voltage and current supplied to the primary coil of the transformer.
A conventional resistance welding apparatus can comprise a switch between the output of the converter bridge circuit and the primary coil of the welding transformer to increase security for an operator using the resistance welding equipment. The security switching element in a conventional power supply unit is provided in the current supply path between the converter circuit comprising the inverter bridge and the primary coil of the transformer. The protective switching element between the output of the converter circuit and the input of the transformer is only closed during the welding process. Consequently, only during the welding process, voltages are applied at the transformer supply lines when the protective switching element is switched on. If no resistance welding is performed the protective switching element switched in series between the converter circuit and the transformer is automatically switched off by a controller of the conventional power supply unit. The protective switching element is formed by an electromechanical switching element comprising a contactor or a relay controlled by a controller of the resistance welding apparatus. The stray or leakage inductivity introduced by the electromechanical protective switching element connected in series between the output of the converter circuit and the transformer causes additional power losses because of skin effects. With increasing switching frequency, the skin effect increases the power losses. With switching frequencies above 1 kHz the leakage inductivity caused by the protective switching element leads to unacceptable reduction of the performance capability of the power supply.
Accordingly, there is a need for a power supply unit providing security for an operator of a resistance welding apparatus providing a minimum power loss when using high switching frequencies.